Aluminum silicon casting alloys



United States Patent 3,128,176 ALUMINUM SILICON CASTING ALLOYS WayneMartin, 19820 Battersea Blvd, Rocky River, Ohio No Drawing. Filed June14, 1961, Ser. No. 116,929 2 Claims. ((11. 75-148) This inventionrelates to new and improved aluminum casting alloys and moreparticularly to aluminum casting alloys adapted to overcome many of theproblems arising in the use of such aluminum alloys in permanent moldcasting and in die casting processes.

With reference first to sand and permanent mold casting, one of thedifliculties which has heretofore been encountered stems from the highshrinkage that takes place in the cast metal during solidification andcooling. Such shrinkage tends to produce voids in the cast metal productand shrinkage cracks which render the product unusable for many of theuses for which it was intended. These shrinkage difficulties increase inproportion to cross-section of the article being cast and thus become aconsiderable factor in the casting of articles of large dimension andweight. Under these circumstances, excessive losses as scrap materiallyincrease the cost of the casting process.

In the field of die casting, problems arise with respect to theinability of the metal completely to flow together during the castingoperation whereby a casting can be secured with voids, discontinuities,surface roughness or other defects. This also results in the formationof a casting of low strength due to failure at the point ofdiscontinuity when the casting is placed under stress. This also resultsin castings which are not pressure-tight because of the voids ordiscontinuities formed therein. Such discontinuities and surfaceroughness also impair the ability to provide a smooth and attractivefinish since such discontinuities in the surface show through and cannoteasily be removed or concealed. It has been found and believed that suchdiscontinuities in die casting of aluminum base alloys result from thepresence of a tenacious oxide skin or layer which forms almostinstantaneously on the surface of the molten metal and that the surfacetension characteristics of this oxide film are such as to resist wettingout by the meeting streams of molten metal flowing through the mold inthe die casting operation. As a result the surfaces tend to separate onefrom the other and sometimes the fluidity is reduced or is so low thatthe metal solidifies before these separations can be forced together bythe pressure of the plunger, with the result that these discontinuitieswill remain in the cast product.

To the present, fluidity has been imparted to the aluminum castingalloys by the addition of silicon. For this purpose, aluminum alloysused for die casting have been formulated to contain more than 7 percentsilicon and preferably an amount of silicon within the range of 8.5 -to13 percent. An aluminum casting alloy with less than 7 percent siliconhas not heretofore been considered suitable for use in die casting.Maximum fluidity is achieved in a formulation containing from 10 to 12percent silicon but at 11.6 percent silicon, the eutectic composition isreached whereby the metal freezes almost instantaneously upon cooling.Thus the problem of discontinuity is aggravated rather than beingdecreased by the addition of optimum amounts of silicon for maximumfluidity.

Another problem often encountered in the casting of aluminum base alloysis often referred to in the trade as soldering to the die, which refersto the sticking of the cast metal product in the die. To preventsoldering, it has been necessary to provide for an iron content in3,128,176 Patented Apr. 7, 1964 the aluminum casting alloy in an amountin excess of 1 percent. Such high amount of iron in the aluminum castingalloy has posed a number of problems having to do with the foregoing inthat the iron reacts in the presence of manganese and chromium, usuallypresent in the aluminum casting alloy to form ,B-iron silicides. Theserepresent hard spots in the casting and they materially lower thefluidity of the molten metal.

It is an object of this invention to produce and to provide a method forproducing aluminum casting alloys wherein the problems of the typespreviously described are materially reduced in the use of the aluminumcasting alloys in the fabrication of products by various of the castingtechniques and it is a related object to produce a new and improvedaluminum casting alloy.

More specifically, it is an object of this invention to produce aluminumcasting alloys which substantially avoid shrinkage problems uponcooling; which embody high fluidity for use in die casting; which enablethe silicon content to be reduced without corresponding loss in fluiditythereby to extend the solidification or freezing range of the metal;which enable reduction in iron content to below the level for harmfulformation of ,B-iron silicides without loss in fluidity, which reducedie soldering even With alloys having conventional iron content, andwhich provide for an unexcepted increase in other physical andmechanical properties of the product cast from the alu minum base alloyincluding shock resistance, impact re sistance and strength.

It has been found that these various improvements can be simply andeffectively achieved by the combination to introduce calcium inelemental form into the molten aluminum casting alloy. The resultssecured are wholly unexpected since the presence ofcalcium in aluminumcasting alloys has heretofore been considered and found to be mostundesirable. Calcium has been defined as an impurity in such alloys andevery effort has been made to reduce the calcium content to below apredetermined level. Usually 0.003 percent has been considered themaximum amount of calcium which could be tolerated in aluminum basecasting alloys.

Calcium is brought into the aluminum casting alloys with the addition ofsilicon which has been used to provide fluidity in the aluminum alloyformulation to be supplied to the die casting art. Such calcium findsits presence in the aluminum alloy in the form of calcium silicides,calcium phosphides, calcium nitrides and the like and, when in thisform, the calcium is still considered to be a harmful impurity whichrequires removal to levels below 0.003 and perferably below 0.001percent.

Thus the presence of calcium introduced as elemental metal in accordancewith the practice of this invention should not be confused with thepresence of calcium in the form of the various silicides, nitrides,phosphides and the like, as previously found in such aluminum castingalloys. The latter are considered impurities and require removal tobelow a certain low level while the addition of elemental calcium hasbeen found materially to improve the characteristics of the aluminumcasting alloy. In accordance with the practice of this invention, thedesired amount of elemental calcium is introduced after the calciumimpurities, as heretofore described, are removed to below the maximumfor these impurities. Removal of the calcium impurities to below 0001percent calcium can be effected by chlorination.

The desirable effects from the addition of elemental calcium can besecured when calcium is provided to be present in the alloy in an amountgreater than 0.002 percent. No further beneficial results are secured bythe addition of calcium in a concentration greater than 0.020 percent.Within this range, it is preferred to formulate the aluminum castingalloy with the addition of elemental calcium to provide for aconcentration of calcium within the range of 0.004 to 0.015 percent andmore preferably about 0.004 to 0.010 percent. The elemental calcium canbe added in the desired amounts directly to the molten alloy duringrefining and preferably after the calcium silicides, calcium nitrides,calcium phosphides and the like have been removed, as by chlorination.

The exact mechanisms by which the calcium operates in the alloy systemto impart the unexpected improvements are not yet fully understood. Thedescribed presence of the calcium in the aluminum casting alloy withinthe amounts described appears to enhance the ability of the aluminumcasting alloy to absorb larger amounts of gases while in the moltenstate. Such absorbed gases are released when the molten metal isconverted to the solid state and the gases remain entrapped in thefrozen metal to provide voids of pinhole porosity thereby to decreasethe density of the cast product but more important to compensate for theshrinkage otherwise taking place during solidification and cooling. Thusthe presence of calcium incorporated in accordance with the practice ofthis invention operates to avoid the porosity and shrinkage problemsheretofore encountered in sand and in permanent mold casting. It ispreferred to grain refine the alloy further to reduce the pinholeporosity effected by the release of gases thereby materially to increasethe strength and other mechanical properties of the aluminum alloycasting and also to increase the pressure-tightness of the casting.These improvements may be due in part from the reduction in shrinkagedefects and in part from the fine pinhole porosity and included gaseswithin the alloy. The addition of elemental calcium appears to impartthese beneficial effects in various of the formulations of die andpermanent mold aluminum casting alloys.

The introduction of calcium in accordance with the practice of thisinvention has also been found to provide unexpected improvements inaluminum die casting alloys. Various reasons are ascribed for theseimprovements. It is believed that calcium in the amounts describedbecomes effective to modify the characteristics of the oxide film formedon the surface of the molten aluminum casting alloy or else to preventits formation whereby meeting streams of molten alloy are more readilyable to wet out one another for substantially complete and immediatemergence whereby the discontinuities heretofore experienced are greatlyreduced and, in some instances, substantially completely eliminated.

Instrumental also is the marked increase in fluidity which is achievedby the presence of the calcium in the amounts described. In a diecasting machine, it appears that the fluidity is increased by a factorof about three when calcium is incorporated in accordance with thepractice of this invention. This marked and unexpected increase influidity greatly enhances the flow of the molten metal in die castingwhereby complete filling of the mold can be achieved more rapidly andmore effectively thereby to permit the fabrication of completely formedcastings. This coupled with the desirable effect on the oxide filmresults in a dramatic decrease in discontinuities of the typesheretofore experienced.

In one particular application for die casting gas regulator valves,rejects of up to 50 percent were experienced. When production wasconverted to the use of the same alloy with elemental calcium added toprovide a calcium content of 0.010 percent, rejects were no longerexperienced because of pressure-tightness. In another particularapplication of aluminum casting alloys for the die casting of washingmachine covers, losses of 50 percent due to discontinuities were reducedto less than 20 percent rejects by the modification of the aluminumcasting alloy by the introduction of elemental calcium to provide acalcium content of 0.006 percent.

The marked increase in fluidity which is experienced by the practice ofthis invention permits reduction to be made in the amount of siliconrequired to provide fluidity suflicient for die casting or otherwisemolding the alloy. Reductions in the silicon content to amounts lessthan 11.6 percent enable the alloy to depart from its eutectic whereby agreater spread between the liquidus and solidus states is obtained. Thusthere is more time in which the metal in the mushy state can becompacted by the pressure of the plunger. Thus, a cold chamber diecasting machine can continue to operate to compact the alloy and toforce the various streams to weld at points where discontinuity mightotherwise form. Beneficial use has been experienced when calcium in theamounts described is incorporated into die casting alloys containingfrom 7 to 13 percent silicon, from 0 to 5 percent copper, and from 0 to0.5 percent magnesium. While alloys containing less than 7 percentsilicon have not been desirable for use as a die casting alloy, diecastings can now be made with aluminum casting alloys formulated tocontain as little as 3 percent silicon when elemental calcium in theamounts described has been incorporated in accordance with the practiceof this invention.

The increase in fluidity, achieved by the practice of this invention,also enables the preparation of an aluminum casting alloy with an ironcontent less than 1 percent without experiencing any increase in diesoldering. It appears that the calcium present in amounts within therange of from more than 0.003 to 0.020 percent markedly reduces diesoldering. The iron content can be reduced to as low as 0.6 percentwithout experiencing difficulties in die soldering when 0.010 percentcalcium is also present. Normally, an aluminum base alloy containingless than 0.3 percent iron would solder to the die so strongly as tomake it impractical to cast the alloy. With the addition of elementalcalcium to provide for a calcium content of 0.006 percent, die solderingof the same alloy containing less than 0.3 percent iron can be reducedto a point where the alloy can be employed as a die casting alloy.

Aside from the foregoing improvements which are unexpectedly secured,the reduction in iron and silicon, made possible by the practice of thisinvention, is instrumental in the further improvement of the alloy fromthe standpoint of shock resistance, impact resistance, and elongation.This is an important advantage particularly where shock resistance andhigh strength are material to the final products.

The following are examples of representative aluminum casting alloysformulated to embody the features of this invention in which elementsother than aluminum are set forth:

Example 1 Element: Amount (percent) Ca 0.005. Cu 0.6 max. Si 11.0-13.0.Mg 0.10 max. Zn 0.50 max. Fe 1.2 max. Mn 0.3 max. Ni 0.50 max. Otherseach 0.20 max.

Example 2 Ca 0.007. Cu 0.6 max. Si 9.010.0. Mg 0.450.6. Zn 0.50 max. Fe1.0 max. Mn 0.35 max. Ni 0.50 max. Others each 0.20 max.

Example 3 Element: Amount (percent) Ca 0.007. Cu 3.0-4.0. Si 8.5-9.5. Mg0.10 max. Zn 1.0 max. Fe 1.0 max. Mn 0.50 max. Ni 0.50 max. Others each0.30 max.

Example 4 Ca 0.006. Cu 9.2-10.8. Si 2.0 max. Mg 0.20-0.35. Zn 0.8 max.Fe 1.2 max. Mn 0.6 max. Ni 0.50 max. Others each 0.20 max.

Example 5 Ca 0.005. Cu 9.5-10.5. Si 3.5-4.5. Mg 0.15-0.35. Zn 1.5 max.Fe 1.2 max. Mn 0.6 max. Ni 0.50 max. Others each 0.20 max.

Example 6 Ca 0.007. Cu 3.0-4.0. Si 7.5-8.5. Mg 0.10 max. Zn 1.0 max. Fe1.0 max. Mn 0.50 max. Ni 0.40 max. Ti 0.25 max. Others each 0.20 max.

Example 7 Ca 0.007. Cu 1.0-2.0. Si 4.5-5.5. Mg 0.40-0.6. Zn 0.50 max. Fe0.8 max. Mn 0.50 max. Ni 0.30 max. Ti 0.25 max. Others each 0.20 max.

Example 8 Ca 0.007. Cu 1.0-1.5. Si 4.5-5.5. Mg 0.40-4.6. Zn 0.20 max. Cr0.25 max. Fe 0.6 max. Mn 0.30 max. Ti 0.25 max. Others each 0.05 max.

Example 9 Ca 0.010. Cu 0.6 max. Si 4.5-6.0. Mg 0.10 max. Zn 0.50 max. Fe0.8 max. Mn 0.35 max. Ni 0.50 max.

Others each 0.20 max.

6 Example 10 Element: Amount (percent) Ca 0.010. Cu 0.30 max. Si4.5-6.0. Mg 0.05 max. Zn 0.30 max. Fe 0.8 max. Mn 0.50 max. Ti 0.25 max.Others each 0.20 max.

Example 11 Ca 0.010. Cu 0.10 max. Si 4.5-6.0. Mg 0.05 max. Zn 0.30 max.Fe 0.8 max. Mn 0.35 max. Ti 0.25 max. Others each 0.05 max.

Example 12 Ca 0.010. Cu 4.0-5.0. Si 2.0 max. Mg 0.05 max. Zn 0.50 max.Fe 1.0 max. Mn 0.50 max. Ti 0.25 max. Others each 0.10 max.

Example 13 Ca 0.010. Cu 4.0-5.0. Si 1.5 max. Mg 0.03 max. Zn 0.35 max.Fe 0.8 max. Mn 0.35 max. Ti 0.25 max. Others each 0.05 max.

Example 14 Ca 0.007. Cu 3.0-4.0. Si 5.5-6.5. Mg 0.10 max. Zn 1.5 max. Fe1.0 max. Mn 0.6 max. Ti 0.25 max. Others each 0.30 max.

The elemental calcium may be introduced in various forms such as in theform of solid calcium, powdered calcium or alloys containing calcium inrelatively high concentration. Introduction is made while the aluminumbase casting alloy is in a molten state and preferably after the otherimpurities including the calcium nitrides, phosphides and silicides,which may previously have been brought in with silicon, have beenremoved.

It will be apparent from the foregoing that I have provided a new andimproved aluminum casting alloy and method for the preparation of samewherein the alloy is greatly improved in its fluidity from thestandpoint of casting by permanent mold or die casting and wherebyfurther improvements can be achieved by the reduction of silicon andiron heretofore found to be detrimental to many of the characteristicsof the casting processes but which have previously been essential forthe purposes of introducing suflicient fluidity for die casting orsufficient resistance to soldering to permit removal of the casting fromthe mold. The concepts of this invention to introduce elemental calciuminto aluminum base alloys has been found also eifective to minimizeshrinkage problems and flow problems, especially from the standpoint ofrapid and complete wet-out to eliminate discontinuities in the surfacesof products molded thereof.

It will be understood that changes may be made in the details of theformulation and methods of addition without departing from the spirit ofthe invention, especially as defined in the following claims.

I claim:

1. An aluminum die casting alloy consisting of essentially of from 3 to13 percent silicon and calcium introduced as elemental calcium in anamount Within the range of 0.004 to 0,015 percent, the balance beingessentially aluminum.

2. An aluminum base die casting alloy consisting essentially of siliconin an amount within the range of 3 to 13 percent, calcium introduced aselemental calcium in an References Cited in the file of this patentUNITED STATES PATENTS 1,387,900 Pacz Aug. 16, 1921 1,657,389 GWyer etal. Jan. 24, 1928 3,078,191 Maeda Feb. 19, 1963 OTHER REFERENCES Logan:A New Combined Degassing-Modification Process, Light Metals, April 1956,pp. 122-125.

1. AN ALUMINUM DIE CASTING ALLOY CONSISTING OF ESSENTIALLY OF FROM 3 TO13 PERCENT SILICON AND CALCIUM INTRODUCED AS ELEMENTAL CALCIUM IN ANAMOUNT WITHIN THE RANGE OF 0.004 TO 0.015 PERCENT, THE BALANCE BEINGESSENTIALLY ALUMINUM.